Effect of Vibration on Forced Convection Heat Transfer for SiO2–Water Nanofluids

In the process of heat transfer, the fluid type and external parameters have a significant impact on heat transfer performance. For this reason, the physical properties, pressure differences, and heat transfer rates of SiO₂–water nanofluids have been experimentally investigated in a straight circular pipe. Experimental results revealed a great difference in physical properties between SiO₂–water nanofluids and purified water. The friction factor of low-volume-concentration nanofluids was slightly increased for laminar flow and tended to be almost independent of the Reynolds number for turbulent flow. The heat transfer coefficient can be enhanced either by adding nanoparticles to purified water or by imposing a transverse vibration on the heat transfer surface. Using these two methods at the same time (compound heat transfer enhancement), heat transfer performance is much better than that with either method alone. The largest increase of about 182% was observed under conditions of compound heat transfer enhancement.     

Experimental Study of Thermo-Physical Properties of Nanofluids Based on γ-Fe2O3 Nanoparticles for Heat Transfer Applications

The main goal of the present study was to prepare and also to investigate the effects of both temperature and weight concentration on the thermo-physical properties of γ-Fe₂O₃/water nanofluids. The γ-Fe₂O₃ nanoparticles were synthesized by laser pyrolysis technique and characterized using TEM, XRD, and EDX techniques. Thermal conductivity, viscosity and surface tension of γ-Fe₂O₃/water nanofluids were investigated within the range of the temperature of 20°C to 70°C for various weight concentrations of nanoparticles (0.5, 1.0, 2.0, and 4.0 wt%). The experimental results show that the thermal conductivity ratio is much higher than of thermal conductivity of base fluid. Thus, the relative thermal conductivity was 59% for a concentration of 4.0 wt% and a temperature of 50°C. Also, it has been observed that the influence of weight concentration of nanoparticles on viscosity was lower at temperatures over 55°C. At standard temperature of 25°C and 2.0 wt.% concentration of nanoparticles, the relative dynamic viscosity was 5.61%. Experimental results show that the surface tension increases with increase of weight concentrations and decreases with increase of temperatures. For a temperature of 70°C and 2.0 wt.% concentration of nanoparticles, the relative surface tension was 46%. The experimental results were compared with data available in literature.     

An Experimental Study of Small-Diameter Wickless Heat Pipes Operating in the Temperature Range 200°C to 450°C

An experimental investigation is reported for medium-temperature, wickless, small-diameter heat pipes charged with environmentally sound and commercially available working fluids. The wickless heat pipes (thermosyphons) studied have many applications in heat recovery systems since their operational temperature range is between 200°C and 450°C. The heat pipes investigated had an internal diameter of 6 mm and a length of 209 mm. The lengths of evaporator and condenser sections were 50 mm and 100 mm, respectively. The working fluids tested were diphenyl based: Therminol VP1 and Dowtherm A. High-grade stainless steel was chosen as the shell material for the heat pipes to provide chemical compatibility between heat pipe casing material and working fluids at elevated temperatures. Thermal resistances of less than 0.4 K/W have been achieved at working temperatures of up to 420°C with an effective thermal conductivity of 20 kW/m-K, which corresponds to an axial heat flux of 2.5 MW/m². Even for such small-diameter heat pipes, the experimental data for the evaporator showed good agreement with Rohsenow's pool boiling correlation.     

The Effect of Alumina–Water Nanofluid on Natural Convection Heat Transfer Inside Vertical Circular Enclosures Heated from Above

Experimental investigation on natural convection heat transfer is carried out inside vertical circular enclosures filled with Al₂O₃–water nanofluid with different concentrations; 0.0%, 0.85% (0.21%), 1.98 (0.51%), and 2.95% (0.75%) by mass (volume). Two enclosures are used with 0.20 m inside diameter and with two different aspect ratios. The top surface of the enclosure is heated using a constant-heat-flux flexible foil heater while the bottom surface is subject to cooling using an ambient air stream. Various heat fluxes are used to generate heat transfer through the nanofluid. The average Nusselt number is obtained for each enclosure and correlated with the modified Rayleigh number using the concentration ratio as a parameter. A general correlation for the average Nusselt number with the modified Rayleigh number is obtained using the volume fraction and the aspect ratio as parameters to cover both enclosures. The results show that the Nusselt number for the alumina–water nanofluid is less than that of the base fluid. This means that using the alumina–water nanofluids adversely affects the heat transfer coefficient compared to using pure water. It is also found that the degree of deterioration depends on the concentration ratio as well as the aspect ratio of the enclosure.     

Local Convective Heat Transfer from Small Heaters to Impinging Submerged Axisymmetric Jets of Seven Coolants with Prandtl Number Ranging from 0.7 to 348

INTRODUCTIoNJetimpingementhasbeenextensivelyemployedintechnicalprocessestoproducerelativelyhighheat/massfluxes.Incomparisonwiththeheat/masstransferratesprovidedbyconventionaltechniqueswithfluidfiowsparalleltotheheat/masstransfersur-face,aremarkableincreaseintransfercoefficientscanbeobtainedinthisfashion.Inmostcasesairisusedastheworkingmedium.Examplesofairjetapp1icationsincludecoolingofturbinebladesandelectroniccom-ponents,annealingofmetallicandplasticsheets,dry-ingoftextilesandpaper,andtem…     

Effective thermal conductivity of Mexican geothermal cementing systems in the temperature range from 28°C to 200°C

The effective thermal conductivity of six Mexican cementing systems used in geothermal well completion were experimentally determined in the temperature range from 28°C to 200°C. Measurements were carried using the classical line-source method. The experimental system was calibrated by measuring the thermal conductivity of standard fused quartz samples. An experimental procedure for preparation of the cement specimen samples was also developed. Results show that thermal conductivity depends on the particular cement system and tends to increase with temperature for most cement systems. Experimental uncertainties of thermal conductivity were less than 4%. From this experimental work, new empirical equations for correlating thermal conductivity with temperature for geothermal cementing samples were obtained.     

Momentum and Heat Transfer Phenomena for Power–Law Liquids in Assemblages of Solid Spheres of Moderate to Large Void Fractions

This work extends our previously reported results for the flow of and heat transfer from expanded beds of solid spheres to power–law fluids by using a modified and more accurate numerical solution procedure. Extensive results have been obtained to elucidate the effects of the Reynolds number (Re), the Prandtl number (Pr), the power–law index (n), and the bed voidage (ε) on the flow and heat transfer behavior of assemblages of solid spheres in the range of parameters: 1 ≤ Re ≤ 200, 1 ≤ Pr ≤ 1000, 0.6 ≤n ≤ 1.6, and 0.7 ≤ε ≤ 0.999999. The large values of bed voidage are included here to examine the behavior in the limit of an isolated sphere. As compared to Newtonian fluids, for fixed values of the Reynolds number and the voidage, the total drag coefficient decreases and the average Nusselt number increases for shear thinning fluids (n < 1); whereas, for shear thickening fluids (n > 1), the opposite behavior is observed. The drag results corresponding to bed voidage, ε = 0.99999, are very close to that of a single sphere; whereas, the heat transfer results approach this limit at ε = 0.999. Based on the present numerical results, simple correlations for drag coefficient and average Nusselt number are proposed which can be used to calculate the pressure drop for the flow of a power–law fluid through a bed of particles, or rate of sedimentation in hindered settling and the rate of heat transfer in assemblages of solid spheres in a new application. Broadly speaking, all else being equal, shear-thinning behavior promotes heat transfer, whereas shear-thickening behavior impedes it.     

Characterization of metal hydrides for thermal applications in vehicles below 0 °C

Metal hydrides promise great potential for thermal applications in vehicles due to their fast reaction rates even at low temperature. However, almost no detailed data is known in literature about thermochemical equilibria and reaction rates of metal hydrides below 0 °C, which, though, is crucial for the low working temperature levels in vehicle applications.Therefore, this work presents a precise experimental set-up to measure characteristics of metal hydrides in the temperature range of −30 to 200 °C and a pressure range of 0.1 mbar–100 bar. LaNi_4.85Al_0.15 and Hydralloy C5 were characterized. The first pressure concentration-isotherms for both materials below 0 °C are published. LaNi_4.85Al_0.15 shows an equilibrium pressure down to 55 mbar for desorption and 120 mbar for absorption at mid-plateau and −20 °C. C5 reacts between 580 mbar for desorption and 1.6 bar for absorption at −30 °C at mid-plateau.For LaNi_4.85Al_0.15, additionally reaction rate coefficients down to −20 °C were measured and compared to values of LaNi₅ for the effect of Al-substitution. The reaction rate coefficient of LaNi_4.85Al_0.15 at −20 °C is 0.0018 s⁻¹. The obtained data is discussed against the background of preheating applications in fuel cell and conventional vehicles.     

Crosslinked poly (isatin biphenyl spirofluorene) membranes for proton conduction over a wide temperature range from −40 to 160 °C

It is desired to develop proton exchange membranes (PEMs) working in a wide temperature range considering the practical working condition of devices using the PEMs as the electrolyte. Herein, a novel polymer of poly (isatin biphenyl spirofluorene) (PIBS) is first synthesized and it is afterwards crosslinked by 1,3-bis(4-piperidyl) propane (P) to fabricate membranes. The membranes can work in a temperature range of −40 to 160 °C after doping with phosphoric acid (PA). The proton conductivity of the PA doped membrane reaches 4.4 × 10⁻³ S cm⁻¹ at −40 °C under 80% relative humidity (RH) and 0.16 S cm⁻¹ at 160 °C without humidifying. We demonstrate the uses of the prepared PA doped PIBS-P membranes as membrane electrolytes in single fuel cells within 100–160 °C under anhydrous condition, and in water electrolytic cells within −20 to 60 °C, respectively.     

Syntheses of LiCoO2 for cathode materials of secondary batteries from reflux reactions at 130–200 °C

Low temperature syntheses of LiCoO₂ cathode materials for Li-secondary battery applications were studied in aqueous solutions by hydrothermal and reflux reactions. By controlling the oxidation potential of the reaction environments, we could synthesize phase pure LiCoO₂ directly from Co(OH)₂ at as low as 130 °C, contrary to the reported hydrothermal reactions at 200–220 °C starting with CoOOH. Having high pH over 15 and an O₂ flow induced the oxidation reaction of the divalent cobalt in Co(OH)₂. The products so obtained have well-crystallized high temperature form of LiCoO₂ with the layered structure as proved by powder X-ray diffraction (XRD) and the Raman spectroscopy data. The particles have a uniform size distribution around 100 nm with well-developed crystallite morphology.